It is known that the use of an electric automobile is advantageous as compared with an automobile using an internal combustion engine, from the viewpoints of energy saving and air pollution. The electric automobile, however, is required to be more improved, particularly, in its driving distance at one charging operation, driving performance such as acceleration performance, and safety in the driving operation. The improvement can be, in part, attained by the improvement of a motor for driving the automobile. It is considered, however, that the improvement can be attained more efficiently by improving an electric battery to be mounted onto the automobile.
The driving distance at one charging operation can be prolonged by mounting a battery having a large electric capacity onto an automobile. Generally, a battery having a large electric capacity is large in its volume and heavy in its weight.
In order to solve the above-mentioned problems, a lithium ion secondary battery is paid an attention. It is known that a lithium ion secondary battery utilizing a lithium cobalt oxide as a positive electrode material and a carbonaceous material (e.g., graphite) as a negative electrode material can give a high voltage in the range of 3 to 4 V and a high energy density in the range of 200 to 300 Wh/L. The energy density of the lithium ion secondary battery is two to three times as much as that of a generally employed lead-acid battery (approximately 100 Wh/L) and 1.5 times as much as that of the nickel-hydrogen secondary battery (160 to 200 Wh/L). Therefore, if the lithium ion secondary battery is employed, the volume and weight of the battery can be greatly reduced. However, it is also important that the lithium ion secondary battery should be operated at the optimum temperature condition to keep the efficiency of charging and discharging operations. Japanese Patent Provisional Publications No. H7-1973 and No. H8-22845 describe a temperature-controlling means for a battery mounted in an automobile which utilizes a circulating water.